1. Field of the Invention
The instant disclosure relates to a memory device and manufacturing method thereof: more particularly, to a vertical transistor for random-access memory and manufacturing method thereof.
2. Description of Related Art
For the mainstream IC processing, the most common transistor is the MOSFET (metal-oxide-semiconductor field-effect transistor). Like typical transistors, the current flows through the channel region of the MOSFET. In particular, n-type MOSFET (nMOSFET, nMOS) is provided with conducting electrons, whereas p-type MOSFET (pMOSFET, pMOS) uses conducting “holes” for operation.
FIG. 1 shows a typical p-type MOSFET (pMOS), which comprises an n-type substrate 1a, a gate 2a, and two spacers 3a. As a source and a drain, a first doping area 11a and a second doping area 12a are disposed on the n-type substrate 1a. An oxide film 13a is disposed on the n-type substrate 1a. The gate 2a is disposed on the oxide layer 13a, and the spacers 3a are disposed on the sides of the gate 2a over the oxide layer 13a. The source, drain, and the gate 2a of the above-described pMOS are arranged horizontally, which occupy more surface of the n-type substrate 1a. Thus, the packing density of the semiconductor element is restricted. In addition, after repeated read or write access operation, electric charge accumulation tends to occur. The threshold voltage Vt becomes more fluid, rendering the pMOS to be less stable.